1. Field of the Invention
The present invention relates to a display device of an optical scan type which is expected to apply to audio-visual equipment provided with a display such as a TV, office-automation equipment such as a personal computer, or optical information processing equipment.
2. Description of the Related Art
In recent days, a matrix type liquid crystal display device has been requested to have larger capacity. That is, as the display device has higher resolution, it is requested that the number of pixels per inch included in the display device be increased from 400.times.600 to 1000.times.1000 or more. Likewise, the size of the display screen has been requested to increase from 10 to 20 inches or more.
To realize such requests, however, the active matrix type liquid crystal display device, in particular, the liquid crystal display device employing thin film transistors each serving as a switching element may have a shortcoming. That is, the wires are made longer and the resistance on the wire is made larger accordingly. As a result, the resistance on the wire and the floating capacitance may bring about a delay of a signal waveform. On the other hand, the simple matrix type liquid crystal display device also has a shortcoming that as the scan lines corresponding to a large screen are increased in number, a voltage ratio of selected pixels to non-selected pixels, that is, a duty ratio cannot be kept proper, thereby degrading the display characteristic.
In order to overcome these shortcomings, a high-resolution optical scan type display device has been proposed in Japanese Patent Laid-Open Application 1-173016, 2-89029, and 2-134617, etc. These types of display devices use a light signal for excluding these shortcomings resulting from the resistance and the floating capacitance on the wire.
The inventors of the present application know that the optical scan type display device is basically arranged to have a pair of substrates opposed to each other and a liquid crystal layer laid between these substrates. These substrates are both made of transparent glasses. On a base substrate, a plurality of optical waveguides are located in a horizontal manner. A plurality of signal wires are located in a vertical manner so that these signal wires crossed at right angles with those optical waveguides, respectively. Each optical waveguide includes a luminous portion at one end. By activating each luminous portion, a light signal is propagated through the optical waveguide connected to the luminous portion. At each crossing point between the optical waveguide and the signal wire, a light switching element is formed on the optical waveguide. The light switching element is made of an optical waveguide layer. One end of the light switching element is vertically laid between the signal electrode and the optical waveguide. The other end of the light switching element is vertically laid between the optical waveguide and a pixel electrode. At the switching element on the optical waveguide, a V-grooved light scattering portion is formed for radiating a light signal propagated through the optical waveguide to the switching element. Further, at each section being formed by the optical waveguide and the signal wire and including the switching element, the pixel electrode is buried on the section area so that the pixel electrode is partially overlapped on the optical waveguide. On the surface of the base substrate, an orientation film is formed in a manner to cover the surface.
In turn, on the inside of the other substrate opposed to the above base substrate, there is formed an opposed electrode. The opposed electrode is made of a transparent conductive material. On the opposed electrode, a light cut-off layer is formed at each location corresponding to each switching element on the base substrate. For covering the inside of the opposed substrate formed as above, an orientation film is formed. A liquid crystal layer is laid and pasted between the base substrate and the opposed substrate and sealed by a sealing member.
The optical waveguide is an ion diffusion type waveguide which is made by diffusion metal ions on the base substrate made of glass. The optical waveguide has a semi-circular section and is buried in the base substrate so that the surface of the optical waveguide is on the same level as the surface of the base substrate. The part of the optical waveguide being located on the same level as the surface of the base substrate is covered by a clad layer formed on the overall surface of the base substrate. This clad layer is made of an SiO.sub.2 film.
Next, the description will be oriented to the operation of the liquid crystal display device of an optical scan type active matrix type arranged as described above.
The luminous portion serves to fire a light scan signal. The light signal is conveyed inside of the optical waveguide. The light is scattered through the light scattering portion provided on the optical waveguide. Part of the scattered light is applied onto the light switching element. The light switching element serves to change its impedance according to the intensity of light applied thereon through the photoconductive effect. The change of the impedance of the light switching element depending on the brightness and darkness of the light leads to controlling flow of current between the signal wire and the pixel electrode. That is, in the light-applied state, the light switching element lowers its impedance so that the signal wire may be electrically connected with the pixel electrode. This connection results in being able to apply a data signal onto the liquid crystal layer laid between the pixel electrode and the opposed electrode through the signal wire. In the dark state, on the other hand, no data signal is allowed to be applied onto the liquid crystal layer between the pixel electrode and the opposed electrode in a manner that the voltage applied onto the liquid crystal layer in the bright state may be maintained.
In place of an electric gate signal, which has been heretofore used for a thin film transistor element, a light signal propagated through the optical waveguide may drive the light switching element. In general, the light switching element included in the optical scan type liquid crystal display device may use as a photoconductive material amorphous silicon hydride (a-Si:H), which may be formed on a relatively large area at a low temperature by means of the CVD method.
The aforementioned structure of the known optical scan type active matrix LCD has some problems. At first, when realizing a large screen, a propagating characteristic is not proper in the optical waveguide for propagating a light signal. For example, consider a screen of diagonally 40 inches. The screen size is about 88.4.times.49.8 cm. Such a large screen needs an optical waveguide of about 90 cm. For the screen of diagonally 20 inches, the screen size is about 44.3.times.24.9 cm. It needs an optical waveguide of about 45 cm, and for the screen of diagonally 80 inches, the screen size is about 132.8.times.74.7 cm, with an optical waveguide of about 130 cm. These sizes are applicable for the aspect ratio 16:9.
Table 1 shows the propagation characteristics in case of changing the propagation loss of the optical waveguide and the length of the optical waveguide.
TABLE 1 ______________________________________ (1-1) Propagation Loss .alpha. (a) 0.2 dB/cm (b) 0.1 dB/cm (c) 0.05 dB/cm (d) 0.02 dB/cm (e) 0.01 dB/em (f) 0.005 dB/cm (g) 0.001 dB/cm (1-2) Intensity Ratio after 45 cm propagation (a) 12.59% (b) 35.84% (c) 59.57% (d) 81.28% (e) 90.16% (f) 94.95% (g) 98.97% (1-3) Intensity Ratio after 90 cm propagation (a) 1.58% (b) 12.59% (c) 35.48% (d) 66.07% (e) 81.28% (f) 90.16% (g) 97.95% (1-4) Intensity Ratio after 130 cm propagation (a) 0.25% (b) 5.01% (e) 22.39% (d) 54.95% (e) 74.13% (f) 86.10% (g) 97.05% ______________________________________
where (a) to (g) of (1-1) correspond to (a) to (g) in each of (1-2) through (1-4), respectively.
The propagation loss .alpha. in the list (1) may be derived by the following expression (1) EQU .alpha.={-10.times.log.sub.10 (P.sub.out /P.sub.in)}/L (1)
where .alpha. is a propagation loss, L is an optical waveguide length, P.sub.in is an intensity of light given when it enters into the optical waveguide, and P.sub.out is an intensity of light given when it gets out of the optical waveguide.
As indicated in Table 1, for example, optically scanning the diagonal 60 inches, it is damped to 22% of the intensity of incident light in the propagation loss of 0.05 dB/cm, thus a light source with high output power is required in order to obtain a sufficient light intensity at the furthermost location from the light source, thereby it is desirable to have the propagation loss equal to or less than 0.02 dB/cm, more preferably 0.01 dB/cm. Similarly, for the diagonal 40 inches, it is desirable to have the propagation loss equal to or less than 0.05 dB/cm or 0.02 dB/cm, and for the diagonal 20 inches, it is desirable to have the propagation loss equal to or less than 0.1 dB/cm.
By the way, for producing the light guide path, the following methods (a) through (d) may be referred.
(a) Pasting an optical fiber on the rear surface of a glass substrate;
(b) Patterning an organic film and an inorganic film;
(c) Using the ion exchange method and the proton exchange method;
(d) Forming a groove in a glass substrate and burying a fiber into the groove or molding a resin in the groove.
Those methods have the following problems.
For the method (a), a transmission loss of the optical fiber is as small as 100 dB/km or less. It means that the optical fiber itself has an excellent transmitting efficiency. However, since the optical fiber is pasted on the rear surface of the glass substrate, scattering of the light signal is brought about until it enters into the light switching element. This results in lowering the utilization efficiency of light in the light switching operation.
For the method (b), it is advantageous in light of the manufacturing process. The propagation loss of the currently available organic material is larger than that of the optical fiber by one digit, so that the propagation efficiency of light is made worse. In case of using an inorganic material in the method (b), it is difficult to obtain as high a propagating characteristic as the optical fiber through the effect of the deposition technique.
Likewise, the method (c) has difficulty in obtaining a predetermined film thickness and being the larger propagation loss.
For the method (d), it is considered that the propagation loss of the optical waveguide formed by molding the resin in the groove is equivalent to that of the organic films of the method (b), thus the propagation efficiency of light is not reasonable. On the other hand, in case of filling a fiber into the grooves and then fixing with a pasting agent, it is expected to obtain sufficient propagation characteristics since the propagation loss of the fiber used is small, but because of the heat resistance, the thermal expansion coefficients, etc., of the pasting agent, the heat history of the resin in the manufacturing process, concretely, that is, because of the shrinkage of the pasting agent through the repetition of the change of the heating temperature of the resin from a high temperature of about 200.degree. to 250.degree. C. to a room temperature or a lower temperature, then it will have a possible danger such that the fiber will be distorted and resulting in increasing the propagating loss. Therefore, the materials having a high heat resistibility of the process heat or more must be used. The above description is also applicable for molding the resin into the grooves.
In addition, in a state that the fiber is filled therein, a surface thereof is rough and it is difficult to form an element thereon. Also, in order to induce the light into the switching element from the optical waveguide, the clad layer of the surface must be removed, and in that degree, the propagation loss of the optical waveguide increases drastically.
Therefore, those methods have difficulty in producing an optical waveguide having an excellent propagating characteristic.
Then, consider the display device having a lot of scan lines. For example, if the number of necessary scan lines is 1125 as in a high-definition TV, a ratio of a selecting/non-selecting time per one scan line is as small as 1/1124 if such a display device is driven by scanning the lines in sequence. However, the known light switching element arranged as described above has difficulty in charging the liquid crystal for a quite short time. Hence, the known light switching element arranged as described above needs to have a more improved characteristic.